I need help in making a new player characters which known as the the Shadow Master. The character is going to be neutral evil. I want the character have shadow based powers and magic. I really want the character to be a cleric of Erebus from Dragon 322. The character is going to multiclass. I am also thinking that another class it might have is wizard (uses shadow and darkness based spells and illusions). Would it be a good idea for the character to be a Shade? The problem is I would like it have the abilities of a rogue.
Clerics most often work best if they don't multiclass, unless it's in a PrC that advances spellcasting. After all, spells are a Cleric's bread and butter, and are just about the most potent class feature there is in D&D.
That also means that taking the Shade template with it's huge LA hit (+4, IIRC) is a very, very bad idea. You lose out on class levels, which are superior to what the template gives you in most every way: the templates powers can be more-or-less emulated with spellcasting, while the loss of HD, BAB, saves, skills, feats etc. isn't made up for by the template.
If you're dead set on playing a race with more of a shade-y feeling, make sure
LA buyoff rules are in effect. If they're not, you might still get away with a single LA+1, although from a pure optimization standpoint, I'd advise against it. Consider taking the Dark template (Tome of Magic), which is good for the cost, and maybe add it onto a Whisper Gnome. Another template worth considering is the Shadow-Walker (Unapproachable East), which grants a bunch of shadow-themed spell-like abilities.
For multiclassing, look into Shadow Adept (Player's Guide to Faerûn), which gives a few options for your spellcasting, making it hard to detect and dispel. Shadowcraft Mage (aka killer gnome) has already been suggested, and can be made to work on a Cleric base - although I haven't tried to do that yet. On a Wizard base, it's potentially one of the most powerful PrCs in the game. Should fit like a second skin thematically. For a more powerful melee fighter, Ruby Knight Vindicator (Tome of Battle) is an excellent choice. If you put some emphasis on the Shadow Hand school, your fighting style will have that shady theme, as well (you might want to build from a Cleric/Swordsage base to maximize this, although Cleric/Crusader provides better synergy). In general, Ordained Champion (Complete Champion) is a pretty powerful option for melee clerics, too, although you'll have to reskin it for a different deity.
Stay away from anything in the Faerûn books that has Shar as a deity, even though the shadow/darkness theme pervades that stuff. It's just underpowered and unsatisfying from a mechanical standpoint.